1. Field of The Invention
The present invention relates to a method and to a signal predistorter circuit for eliminating the nonlinearities in the input/output characteristics of nonlinear stages, in particular for eliminating the third-order nonlinearities. More particularly, it relates to a method and to a predistorter circuit for external optical modulators of television signals.
2. Description of the Related Art
The distributing of television signals via CATV (CAble TeleVision) apparatus can be performed with optical fibres. Modulation of the optical signals can be achieved directly, by acting upon the optical source, usually a laser, or indirectly, via an optical modulator.
An optical modulator which makes it possible to perform the amplitude modulation of an optical signal, with a radiofrequency (RF) modulating signal (also known as the external signal) having very high frequencies (as for example other carriers of television channels which conventionally range from 40 to 860 MHz), consists for example of a device based on an interferometer of the Mach-Zehnder type constructed in lithium niobate (LiNbO.sub.3).
A characteristic which is required of an external modulator is linearity of modulation, very important for use in CATV apparatus with analogue type transmission, in particular when the modulating signal is not a single television channel, but consists of many television channels, for example from 40 up to 80 channels.
The electro-optical characteristic (optical output power versus radiofrequency input voltage) of modulators of this type (Mach-Zehnder interferometer) is typically nonlinear. To limit the distortion of the signals it is expedient to operate the modulator near as linear a stretch as possible of the characteristic.
For this purpose, to an electro-optical modulator of the Mach-Zehnder interferometer type there is applied a radiofrequency (RF) modulating signal to an electrode RF and, to the same electrode or to a second electrode, a d.c. supply voltage which determines the working point of the modulator.
A modulator of this type is for example marketed by the Applicant under the reference PIR PIM1510.
The modulating signal, applied to the RF input, consists, for example, of the set of modulated carriers of the television channels to be distributed to the user.
In the case of a Mach-Zehnder modulator the profile of the characteristic can be approximated by a sinusoid, and it is advantageous that the modulator operate near the point of inflection of the sinusoid, to correspond with an applied working point VQ voltage.
The modulation characteristic of the Mach-Zehnder modulator, referred to the working point, can be expressed through the relation: EQU P.sub.u =K.sub.z sin.beta.
where:
P.sub.u is the optical output power PA1 K.sub.z is a coefficient which depends on the characteristic of the Mach-Zehnder modulator. PA1 .beta.=.pi.V/V.sub.x is the modulation index of the modulating signals, expressed in radiands PA1 V is the variation in the applied voltage, with respect to the working voltage VQ PA1 V.sub.x is a constant. PA1 the value of the voltage, the said V.pi., which represents the voltage variation to be applied to the RF (radiofrequency) electrode so as to carry the optical power from the maximum value to the minimum value; PA1 the value of the voltage VQ which should be applied to the supply electrode, so as to cause the working point to correspond to that of inflection of the characteristic with sinusoidal profile, i.e. with odd symmetry. In such a case the distortions of even order (comprising the second harmonic of the applied signals) vanish and the distortions of odd order take on a well defined value. PA1 delivering the said signal to at least one first distorter circuit; PA1 biasing the said at least one first distorter circuit; PA1 distorting the said signal in the said at least one first distorter circuit; PA1 characterized in that the said phase of biasing the said at least one first distorter circuit comprises the phase of maintaining a substantially constant preset voltage value on varying the amplitude of the said signal at the said at least one first distorter circuit. PA1 delivering the said signal to at least one second distorter circuit; PA1 biasing the said at least one second distorter circuit; PA1 distorting the said signal in the said at least one second distorter circuit; PA1 combining the said signals distorted by the said at least one first and one second distorter circuit. PA1 measuring the bias voltage of the said at least one first distorter circuit; PA1 comparing the said measured bias voltage with a preset reference voltage; PA1 producing a control signal representative of the said comparison; PA1 controlling the said bias voltage in response to the said control signal in such a way that the said bias voltage has a substantially constant preset voltage value on varying the amplitude of the said signal. PA1 measuring, in a portion of the circuit, an electrical quantity representative of the said bias voltage of the said at least one first distorter circuit; PA1 calculating the value of the said bias voltage as a function of the said electrical quantity representative of the said bias voltage; PA1 comparing the said calculated bias voltage with a preset reference voltage; PA1 producing a control signal representative of the said comparison; PA1 controlling the said bias voltage in response to the said control signal in such a way that the said bias voltage has a substantially constant preset voltage value on varying the amplitude of the said signal. PA1 an input terminal, able to receive an electrical input signal of variable amplitude; PA1 an output terminal, able to send out an electrical signal to correspond with the said input signal; PA1 a nonlinear element, interposed between the said input terminal and the said output terminal, having a preset relation between an input signal and a corresponding output signal; PA1 a bias circuit electrically connected to the said nonlinear element; characterized in that the said bias circuit comprises means of applying a preset voltage to the said nonlinear element, the said preset voltage being substantially constant on varying the amplitude of the said input signal. PA1 a circuit for measuring the bias voltage of the said nonlinear element; PA1 a generator of a preset reference voltage; PA1 a circuit for comparing between the said measured bias voltage and the said preset reference voltage able to deliver a signal representing the said comparison; PA1 a circuit for controlling the said bias means in response to the said signal representing the said comparison. PA1 means of calculating the bias voltage of the said nonlinear element by measuring a voltage representing the said bias voltage of the said nonlinear element; PA1 a generator of a preset reference voltage; PA1 a circuit for comparing between the said measured voltage and the said preset reference voltage able to deliver a signal representing the said comparison; PA1 a circuit for controlling the said bias means in response to the said signal representing the said comparison. PA1 a predistorter circuit to which is applied a modulating signal comprising at least one biased nonlinear element; PA1 an electrooptical modulator, able to output a modulated optical signal, having an electrical input to which the predistorted modulating signal is applied and having an optical input; PA1 a radiation source connected to the optical input of the said modulator; PA1 characterized in that it comprises means of biasing the said at least one nonlinear element of substantially constant preset voltage on varying the amplitude of the said signal.
This characteristic with sinusoidal profile is distinguished by two values:
For example, in the case of a Mach-Zehnder modulator of the PIR PIM1510 type, produced by the Applicant, the aforesaid voltages can take the following values: EQU V.pi.=4.3V and VQ=0.7V.
The value of the voltage VQ of the working point is not constant, but varies over time (for example through the build-up of static charge in the LiNbO.sub.3) and also with temperature.
Hence the value of the working voltage has to be continually adapted, using as information, for example, the presence and magnitude of the distortions of even order, or of the second-order intermodulation products, referred to overall as CSO (Composite Second Order).
In the case also of the operation of the modulator at the aforesaid working point, which minimizes the distortions of even order, the nonlinearity in the input/output characteristic causes residual distortions of odd order (mainly of third order) which get translated into intermodulation products or CTBs (Composite Triple Beats) and cross modulation (XMOD), which impair the quality of the signals reaching the user. Thus, the quality of television channels, with amplitude modulation of the carrier, is greatly influenced by the presence of the aforesaid intermodulation products, the overall level of which has to be maintained sufficiently low, for example over 65 dB below the level of the picture carrier of each channel, as to obtain good quality of the signals distributed to the users.
To limit these distortions to some extent it is expedient to chose a not too high depth of modulation for the modulating signals (where depth of modulation is to be understood as the maximum value, expressed as a percentage, of the modulation index .beta.), for example around 3.5% or 4% per channel so as to operate as close as possible to the linear portion of the characteristic.
For the purpose of limiting the distortion introduced by the nonlinearity in the characteristic of the modulator, the maximum percentage usage of the modulator is usually about 40%. Thus, the maximum number of channels which can be applied, bearing in mind the aforesaid requirements relating to noise characteristics, is derived by considering that the various carriers, being uncorrelated, sum together statistically in phase quadrature; therefore, the sum of the channels is to be considered in terms of power. Hence, the maximum number of channels which can be applied to the modulator so as not to exceed the aforesaid percentage is around 100.
To reduce the residual distortions, and in particular the third-order distortions, the technique has been suggested of predistorting the modulating signals through the use of nonlinear elements, in such a way that the predistortion is compensated by the subsequent distortion on the part of the modulator. For this purpose a distorter circuit, with an input/output characteristic in respect of the radio frequency signals which is the inverse function of the input/output characteristic of the modulator, can be placed ahead of the modulator.